Nutrient Uptake

Hey Gang!

Quick question about nutrient uptake in DWC:

Does anyone know if plants consume the nutrient solution as a whole, or are some nutrients consumed faster than others.

For example, if a plant consumes a gallon of water, would it also consume 100% of the total dissolved solids in that gallon? Or can it consume something like nitrogen faster than magnesium and over time throwing off the concentration of nutrients in the solution.

I haven’t been able to find any information on this so if anyone has links to an article that would be awesome.

- El Fuego

A plant will use more of one thing than something else which is why you should change with fresh nutrient every 10 days or so topping off in between changes.

Here's a study that was done on tomatoes. The principles are the same for cannabis.

Plants interact with the res on an ad hoc basis. They exchange the chemicals that we use as nutes.

Re. how often to change a res? Some growers change every week, some every two weeks. There's a video on the Jack's website where they discuss changing every week or maybe every other week or once a month. I've read numerous postings by growers with years of experience that they will do a res for veg, change the res for the "flower nutes" and use that res for the rest of the grow.

Nutrient companies, generally, recommend changing every week. That's not shocking since they know what's in the nutes, because they want their customers to have excellent outcomes, and because they have a direct financial incentive for customers to change nutrients frequently.

There are a lot of variables to take into account. I have a res with 28 gallons of nutes. It's silly to think that I'd dump 28 gallons of nutes after the seedlings have been in the res of a week. Even as the plants mature, I don't necessarily change every week. I'm more inclined to swap out the res every two weeks. On the other hand, someone with a plant in late flower in a 3 gallon bucket will have a very different schedule.

A lot of rules that I see growers following, and I'm active on four cannabis forums, is done "because that's how it's always been done" and/or that's something that's worked well in the past, so we keep doing it. No complaint with that. Speaking metaphorically, once we get our wheel set up to do what we want, there's little reason to reinvent it but we should understand that other people might be better served by a different wheel.

I've attached the only research that I've seen on this issue. Perhaps you'll find it of value.

xtsho said:

A plant will use more of one thing than something else which is why you should change with fresh nutrient every 10 days or so topping off in between changes.

Here's a study that was done on tomatoes. The principles are the same for cannabis.

Characterizing nutrient uptake kinetics for efficient crop production during Solanum lycopersicum var. cerasiforme Alef. growth in a closed indoor hydroponic system

A balanced nutrient supply is essential for the healthy growth of plants in hydroponic systems. However, the commonly used electrical conductivity (EC)-based nutrient control for plant cultivation can provide amounts of nutrients that are excessive or inadequate for proper plant growth. In this...

journals.plos.org

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Thanks for posting this.

Plants never use all the nutes at the same rate they demand different salts at different times and the balance of the whole is always changing, if you found a nutrient where it would be used evenly all the time, it would be the best nutrient in the world by far

Yes water and different nutrients are absorbed at different rates. You can watch ypur ec to see if the plants drinking just water or eating food

This is also why long term topping off is not a good idea. If you are using say 3 gal of water in your dwc, and every time a gal is gone you mix nutrients and add that gal back to your res. Eventually you will end up with an abundance of 1 thing and not enough of another. I top of thru the week, do a total change every Sunday

Hi, newbie here, well not newbie, but new to DWC & LED, previously HPS & compost.
Been reading a lot, and this seems like quite an active message board.. I've not started yet, but I'm leaning more toward if pH and EC is in tune, (seems a much lower EC is better for DWC?) with temps and RH, and rez temp, a weekly flush does seem like a waste...

If we assume a 3 month turn-around from seed to chop, maybe only 2 or 3 equaly spaced rez changes are needed, unless theres a serious unbalance of some sort?
all things being equal?

This is a good read. The second paragraph under the Stage of Growth heading is especially germane.

There's also good information about nutrient solution management.

futurebanjo said:

I've not started yet, but I'm leaning more toward if pH and EC is in tune, (seems a much lower EC is better for DWC?) with temps and RH, and rez temp, a weekly flush does seem like a waste

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that goes against what you are doing then. if everything is dialed-in, wouldn't yuo want your nutes to be as well?

rkymtnman said:

that goes against what you are doing then. if everything is dialed-in, wouldn't yuo want your nutes to be as well?

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If "dialed in" = providing nutrients to the plants so that they are at the sufficiency level is assumed when the res is brand new. In contrast, it's very hard to determine if the plants are getting sufficient nutes as the nutes in the res are taken up. A Bugbee paper from 2004 provides info about the weaknesses of the common methods of measuring chemical levels in the res. He recommends a diluted refilling solution (1:3).

This page provides a variety of approaches.

I might be mistaken, forgive me if so, but I thought if EC is good and PH is good then you can just top up the rez with water and then add nutes to lower the ph, as the PH will naturaly slowly creep up?
my tap water is about ph7.2/4, EC pretty much zero. it's 'soft' water so I've purchased soft water hydro nutes, I guess they have added cal/mag but i dunno.

The nute chart for my A/B grow and A/B flower seem really overcooked, suggesting EC of over 2.0 at peak, but I'm reading that with DWC an EC of 1.0 to 1.5 is more gentle and less agressive.

Delps8 said:

If "dialed in" = providing nutrients to the plants

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i meant how he/she said ph, ec, rh, temps would all be on point. then why would you want a sub par nute solution?

DTW is really the best option for nute delivery IMO. same ec, same ph every feed.

futurebanjo said:

but I thought if EC is good and PH is good then you can just top up the rez with water

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you really can do it 2 ways: top off with water daily and dump at 7 or 10 days

or top off with nute solution and go a whole grow without changing a res.

i only do a few plants so nute cost is not much

Thanks, yes, sorry for confusing this thread.
I was meaning why not use both tools - if ph gets to high (alkaline), add fert (+acid), if it gets too low add more water to dilute and raise ph. and maybe use PH+ or PH- solution in an emergency.

Assuming EC stays within range, which for DWC the general internet advice the EC should be quite a bit lower than the manufactuers reccomend, lol!

rkymtnman said:

i meant how he/she said ph, ec, rh, temps would all be on point. then why would you want a sub par nute solution?

DTW is really the best option for nute delivery IMO. same ec, same ph every feed.

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I don't want a "sub par" nute solution. The most direct way to know that my solution is "sup par" is to wait until there are signs off insufficiency or toxicity. I'd rather not do that.

Some folks are happy swapping out nutes because the earth has gone around the sun seven times. If that works for a grower, that's great but I question that because it's a "one size fits all approach" and, to me, that's a bug not a feature.

I want to learn why nutrient solution should be changed so I can figure out a better/easier/faster way to do things. The Bugbee paper provides some insight and the two pages I've linked to shed some light on the issue, as well.

Delps8 said:

I don't want a "sub par" nute solution

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sorry dude but you are translating me wrong. i meant you as in the guy that was asking the ?, not you as in you. got it?

futurebanjo said:

Thanks, yes, sorry for confusing this thread.
I was meaning why not use both tools - if ph gets to high (alkaline), add fert (+acid), if it gets too low add more water to dilute and raise ph. and maybe use PH+ or PH- solution in an emergency.

Assuming EC stays within range, which for DWC the general internet advice the EC should be quite a bit lower than the manufactuers reccomend, lol!

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The plant doesn't care about pH - the key is to keep pH in range so that nutrients are taken up. I've attached an example of a nutrient chart. From what I've read, they're all somewhat (in)accurate. "optimal" pH range in hydro is 5.8.

Yes for sure, thats exactly what I mean, so set ph at say 5.6 for arguments sake, and then let it slowly creep up to ~6.4 naturaly, and then either add more fresh water or nutes to bring it back down, like a cycle. (within safe limits), EC permitting.

I'm just struggling to grasp the relationship between PH/EC and diluting the res with tap water or adding more ferts as a control lever.

EDIT: and how dumping 100l of nute rich water would help

rkymtnman said:

sorry dude but you are translating me wrong. i meant you as in the guy that was asking the ?, not you as in you. got it?

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I understood that.

I was responding to the use of the expression "sub par".